I was trying to test a page using the public WPT instance . I wanted to block out several beacon related domains and thus listed them, space delimited, in the block tab. The test completed but the blocked domains specified were still fetched for beacons by WPT and showed up in the waterfall. Our private WPT instance does not have this issue and blocks out the beacon domains correctly.
Can someone comment on their having successfully tested blocked domains using the public WPT instance?